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Misfire only at idle.

viperbluelx

NAXJA Forum User
Location
Ringgold, GA
I have a 98 4.0 and it has a misfire at idle only after its warmed up. It runs perfect above idle. Already has new plugs and wires and I wrapped the injectors in heat wrap. Any ideas? Oh yeah no codes.
 
I have a 98 4.0 and it has a misfire at idle only after its warmed up. ....

Are all your tune-up parts fresh within the last 36,000 miles ? Are you using good quality parts or the cheapest crap they sell ? Are you using Champion spark plugs ? Tested your coil ? Tested your fuel injectors ? Cleaned the Throttle body and Idle Air Controller ? Used some fuel injector cleaner recently ? Tested your fuel pressure ?

The Jeep In-Line 6 cylinder engine isn't the smoothest idling engine out there, are you sure it not just fine ?

Head or head gasket problems are not a very likely cause, unless you have excluded all the more common and logical causes.
 
Sounds exactly like the nightmare I had with my 01 this past month. I replaced the plugs, coil, cam sensor, and screwed with a bunch of little stuff. It had good compression. Finally I gave in and swapped the head, head gasket, etc. Never ran smoother.

Like I said, check out the basics, but theres only 3 things you need, and one of them is screwed up. Fuel or spark (delivery or timing) and compression
 
A vacuum reading would be a good start. If the problem is mechanical, as RCP suggests above, it will show up strongly in vacuum reading. Here's a nice illustration of the possible readings on an old-fashioned carbureted engine. It still mostly applies, except that I've found fuel injection problems will generally not show much on a vacuum gauge. That makes it doubly useful, since a good vacuum reading on a lousy running engine points to injectors.

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm
 
Went offroad Sunday and 2 times it wouldn't start back without cooling off 10 minutes or so, it would just spin over and not start. Then I got a code P0320 so I'm thinking the crank sensor is starting to go out.
 
Well I replaced the crank sensor and Ran seafood through the intake. Still has a misfire at idle in gear and especially when the a/c compressor is running. Then tonight it has developed a random miss going up hills on the interstate. Can a bad coil or cam sensor do this? It feels just like a bad wire so maybe I should replace them now.
 
Did you replace the distributor cap and rotor? May as well finish the tuneup. Not related to the P0320, but good basic strategy.

I would recommend removing both of the connectors in the wiring for the crank sensor. There is one on the sensor itself and another
in the engine bay near the firewall/#6 injector. Get a can of electrical contact cleaner (available anywhere) and thoroughly spray the
connector. Then add a dab of dielectric grease to the connector and reconnect.

The above can remedy a "contact" issue along the wiring route.
 
Problem identified. :D
No, there is nothing wrong with doing that.

As for the issue test the TPS, MAP sensor and coolant sensor. Check your fuel pressure maybe. Do a compression test.
 
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